1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for recording/reproducing information onto/from optical disks and an optical disk unit used by such apparatus, and in particular to a stack type optical disk unit and an optical information recording and reproducing apparatus using the disk unit.
2. Description of the Related Art
In recent years, optical disk memory systems have attracted attention as allowing significantly higher recording density and larger capacity as compared with conventional magnetic recording and it is now being studied in various places.
It is well known that optical disks have features in large capacity and interchangeability which are not found in conventional magnetic disks. As a result of recent performance improvement of magnetic disks, however, magnetic disk products have been announced having a capacity of approximately 750 MB with a 5.25 inch disk and having a capacity of approximately 250 MB with a 3.5 inch disk.
By comparing memory capacities of optical disks with those of magnetic disks, the following conclusions can be drawn:
(1) those disks are nearly equal in line recording density or magnetic disks are slightly superior in line recording density to optical disks;
(2) optical disks are approximately several to ten times larger in track density than magnetic disks; and
(3) the number of recording faces is one for optical disks, whereas it is ten or more for magnetic disks. (Although each optical disk has two recording faces, its online memory capacity is equivalent to one face because optical disks must be turned over.)
From the foregoing, it is apparent that optical disks have no advantage over magnetic disks with the exception that optical disks are superior in track density to magnetic disks. In particular, the difference in number of recording faces is decisive. When optical disks are compared with magnetic disks in apparatus memory capacity, optical disks have little advantage over magnetic disks in the present situation.
A conventional optical information recording and reproducing apparatus for recording/reproducing information onto/from both faces of an optical disk is shown in Japanese Patent Application No. 63-319072, for example.
FIG. 8 is a configuration diagram of this conventional optical information recording and reproducing apparatus which has been developed and proposed by MATSUSHITA. Numeral 51 an optical disk, 17 a motor, 52 a stationary optical section for emitting a collimated beam 53, and 7 a beam distribution section for selectively distributing the collimated beam 53 of the stationary optical section 52 to movable optical sections 34 and 35. Numerals 34 and 35 denote movable optical sections for focusing the laser beam 53 onto signal recording recording tracks located on both the recording faces of the optical disk 51. Numerals 36 and 37 denote linear motors for transporting the movable optical sections 34 and 35 in the radial direction of the optical disk 51 and for searching a target track. Numerals 38 and 39 denote rails for guiding the linear motors 36 and 37. Numeral 18 denotes a laser, numeral 19 a collimation lens system for shaping the laser beam emitted from the laser 18 into a parallel beam having a circular section, numeral 21 a polarization beam splitter for reflecting the beam, which is reflected by the optical disk 51, toward a photodetector 23 for signal detection instead of returning the beam to the laser 18, numeral 23 a photodetector for receiving the beam reflected by the optical disk 51 and for detecting a servo signal and a reproduced signal, numeral 24 a head amplifier for amplifying and processing a signal outputted from the photodetector 23, numeral 40 a variable phase plate for providing the collimated beam 53 with a constant phase rotation, numeral 41 a polarization beam splitter, numeral 42 a quarter wave longth plate (.lambda./4, .lambda. is a wave length of a laser beam), numeral 43 a total reflection prism, and numeral 44 denotes a .lambda./4 plate. Numerals 45 and 46 denote objective lenses for focusing the collimated beam onto a track of the optical disk 51, numerals 47 and 48 rectangular prisms, numerals 49 and 50 actuators for moving the objective lenses 45 and 46 for the purpose of focusing or tracking. Numeral 53 denotes a collimated beam, numeral 105 a servo error signal detected by the head amplifier 24, and numeral 104 a reproduced signal supplied from the optical disk 51.
Operation of the optical information recording and reproducing apparatus configured as heretofore described will hereafter be described. The collimated beam 53 of the laser 18 collimated by the laser 19 is incident upon the optical distribution section 7. Assuming now that data are recorded and reproduced by the movable optical section 35, the collimated beam 53 is provided with phase rotation of .lambda./2 by the variable phase plate 40, totally reflected by the polarization beam splitter 41, and applied to the rectangular prism 48 of the movable optical section 35 via the rectangular prism 43 and the .lambda./4 plate 44. The collimated beam reflected by the rectangular prism 48 is focused onto a surface track of the optical disk 51 by the objective lens 46. By applying photoelectric conversion to the reflected beam, which is supplied from the optical disk 51, in the photodetector 23 and driving the actuator 50 by the servo error signal 105 of the head amplifier 24, the objective lens 46 always focuses the laser beam onto a track and performs tracking. If the laser of the stationary optical section 52 is modulated in intensity with the recording power level by a data signal, data are recorded onto the corresponding track.
When data are recorded or reproduced by the movable optical section 34, the variable phase plate 40 operates with O phase to transmit the collimated beam 53 as it is through the polarization beam splitter 41. The collimated beam transmitted through the .lambda./4 plate 42 is reflected by the rectangular prism 47 of the movable optical section 34 and focused onto a track, which is located on the reverse of the optical disk 51, by the objective lens 45. In the same way as the movable optical section 35, the objective lens 45 drives the actuator 49 by using the servo error signal 105 and thus focuses/tracks the laser beam onto the track.
When the recording and reproducing face of the optical disk is switched from the obverse to the reverse or from the reverse to the obverse by the optical distribution section, however, it takes processing time of approximately several to 100 milliseconds to perform focus servo to a track, then tracking servo pull-in, and track search in the above described configuration. This results in a problem in that the throughput of the apparatus cannot be raised by dealing with the stack type recording face in the same way as a magnetic disk cylinder to reduce the number of search times.
Further, in case recording and reproducing of one recording face is first performed and recording and reproducing of the next recording face is then performed as the processing of the stack type recording faces, servo and track search processing similar to that described above is necessary at the time of transfer to the next recording face. This results in a problem that information cannot be continuously recorded/reproduced.
On the other hand, the above described problems can be solved by providing every recording face with an optical head and a processing circuit system including an independent laser light source. However, problems of increased size, complexity and cost of the apparatus are posed.